1. Field of the Disclosure
The present invention relates generally to power supplies, and in more particular introduces a new topology for the off line switched mode power supplies.
2. Background
Switch mode power supplies are used in a wide variety of household or industrial appliances that require a regulated direct current (dc) voltage for their operation. There are a wide variety of known power supply topologies using PWM (pulse width modulation) or PFM (pulse frequency modulation) control to regulate output voltage.
One type of power supply topology is a resonant switched mode power supply. Resonant switched mode power supplies with PFM control have some advantages, which include having sinusoidal waveforms and intrinsic soft switching. Resonant switched mode power supplies can also operate at higher switching frequencies with low switching loss, utilize smaller magnetic elements, which therefore require smaller packaging, and still operate with high efficiency. Since resonant switched mode power supplies generally do not have waveforms with sharp edges (e.g., waveforms having high di/dt or dv/dt) EMI performance is improved, which therefore enables the use of smaller EMI filters. The output of a resonant switched mode power supply is often achieved by sensing the output and controlling power supply in a closed loop by varying the switching frequency.
High frequency (HF) transformer isolated LLC converters are a type of resonant switched mode power supply, which utilizes the resonance between two inductors and a capacitor. HF transformer isolated LLC converters are popular for a number of reasons, which include the fact that savings on cost and size can be realized by utilizing the magnetizing and leakage inductance of the transformer as at least a part of the resonance component of the LLC converter. In addition, HF transformer isolated LLC converters can achieve good stability when they are operated at above resonance with zero voltage switching, which results in less switching loss and increased efficiency. Furthermore, HF transformer isolated LLC converters can achieve output regulation in a narrow band of frequency control because of their negative and high slope gain characteristic when operating above resonance.
HF transformer isolated LLC converters usually include an ac-dc front end stage to convert an ac signal received from a low frequency (60 or 50 Hz) ac network to a dc input received by the resonant converter stage. Off-line converters are generally required to with a near unity power factor to minimize the input Volt-Ampere consumption. A Power Factor Correction (PFC) front-end stage is interfaced with the ac network to achieve the near unity power factor. One of the most popular topologies for an ac-dc front-end PFC is a boost converter. A PFC boost converter operating in CCM (Continuous Conduction Mode) control with an inner current loop shapes the input current to follow the input voltage to achieve the near unity power factor. However, when operating in DCM (Discontinuous Conduction Mode), which occurs when current pulses through the inductor drop to zero before the end of each switching cycle, the average of each current pulse would naturally follow the sinusoidal waveform of input voltage without necessity of any extra current control loop.